bot_autodifficulty_threshold_high <number>
The game scores every player by how much they contribute (kills, objectives, etc.). When a bot scores this many points above the human average, the game automatically lowers that bot's difficulty so matches stay competitive.
Examples
bot_autodifficulty_threshold_high 5 - Default. A bot has to be 5 points above the human average before it gets nerfed.
bot_autodifficulty_threshold_high 10 - Bots have to dominate by 10 points before they get nerfed.